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    Subject[PATCH 1/3] sched: set group_imb only a task can be pulled from the busiest cpu
    Date
    When cycling through sched groups to determine the busiest group, set
    group_imb only if the busiest cpu has more than 1 runnable task. This patch
    fixes the case where two cpus in a group have one runnable task each, but there
    is a large weight differential between these two tasks. The load balancer is
    unable to migrate any task from this group, and hence do not consider this
    group to be imbalanced.

    Signed-off-by: Nikhil Rao <ncrao@google.com>
    ---
    kernel/sched_fair.c | 10 +++++++---
    1 files changed, 7 insertions(+), 3 deletions(-)

    diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
    index a171138..de8a6a0 100644
    --- a/kernel/sched_fair.c
    +++ b/kernel/sched_fair.c
    @@ -2378,7 +2378,7 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
    int local_group, const struct cpumask *cpus,
    int *balance, struct sg_lb_stats *sgs)
    {
    - unsigned long load, max_cpu_load, min_cpu_load;
    + unsigned long load, max_cpu_load, min_cpu_load, max_nr_running;
    int i;
    unsigned int balance_cpu = -1, first_idle_cpu = 0;
    unsigned long avg_load_per_task = 0;
    @@ -2389,6 +2389,7 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
    /* Tally up the load of all CPUs in the group */
    max_cpu_load = 0;
    min_cpu_load = ~0UL;
    + max_nr_running = 0;

    for_each_cpu_and(i, sched_group_cpus(group), cpus) {
    struct rq *rq = cpu_rq(i);
    @@ -2406,8 +2407,10 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
    load = target_load(i, load_idx);
    } else {
    load = source_load(i, load_idx);
    - if (load > max_cpu_load)
    + if (load > max_cpu_load) {
    max_cpu_load = load;
    + max_nr_running = rq->nr_running;
    + }
    if (min_cpu_load > load)
    min_cpu_load = load;
    }
    @@ -2447,7 +2450,8 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
    if (sgs->sum_nr_running)
    avg_load_per_task = sgs->sum_weighted_load / sgs->sum_nr_running;

    - if ((max_cpu_load - min_cpu_load) > 2*avg_load_per_task)
    + if ((max_cpu_load - min_cpu_load) > 2*avg_load_per_task &&
    + max_nr_running > 1)
    sgs->group_imb = 1;

    sgs->group_capacity =
    --
    1.7.1


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